Chick embryonic spinal cord cells maintained in cell culture produce a growth inhibitor. This newly recognized low molecular weight substance interferes with myoblast myogenesis and fibroblast proliferation in vitro. Studies in progress aim to purify and characterize this substance and to determine its method of action as well as to define the types of cells susceptible to its action. Exploring the origin, action and role of this substance made by cells of the nervous system during the course of growth and development is one of this project's major long-range goals.